The residues of many industrial processes contain soluble salts which make the disposal of these residues more difficult and expensive, insofar as these soluble salts run the risk of being leached out and contaminating the subsoil of the disposal site.
Particularly significant cases of such salts are the potassium salts.
Potassium chemistry is very diverse and is involved in many applications. Potassium chloride in particular (potash or potassium muriate) represents a market of more than 50 million tonnes per annum. Significant resources that have been identified are located for example in Canada, in the CIS and in Europe. The growing worldwide requirements for fertilizer in particular are responsible for a constantly increased demand for potassium resources. Potassium hydroxide, obtained mainly by electrolysis of potassium chloride, is also experiencing considerable expansion, especially in the electronics field, including the manufacture of photovoltaic cells. Generally, the applications of potassium chloride make it necessary to have a high-purity product. Potassium chloride contents of greater than 90%, or 95%, or even 99% are frequently required. Purities greater than 99% are generally necessary when the potassium chloride is subjected to electrolysis in order to produce potassium hydroxide. This is particularly the case when the potassium hydroxide is intended for electronics applications.
It would therefore be desirable to have processes that make it possible to recycle the soluble salts contained in some industrial residues, in particular potassium chloride.
In U.S. Pat. No. 3,647,395, a process is described for recovering alkali metal salts contained in the gases emitted by cement production furnaces. In this process, the vapours of alkali metal salts contained in the gases emitted are condensed and then added to water with the dust from the flue gas. The aqueous solution obtained is separated from the insoluble particles. The latter are then subjected to a succession of dissolving and separating steps. The aqueous solution finally obtained is subjected to a crystallization of the soluble salts. However, this process, which requires a large number of dissolving and separating steps, is complex and does not make it possible to separately regenerate said soluble salts.